Natural prostaglandin is a local hormone which is produced in vivo mainly by the inner walls of arteries and biologically it works as an importent factor to control the cell function in a living body because of its strong physiological activities such as platalet aggregation inhibiting activity and vasodilating activity. Attempts have, therefore, been made to use it directly as pharmaceuticals (P. J. Lewis, J. O. Grady et al., "Clinical Pharmacology of Prostacyclin", Raven Press, N.Y., 1981). However, since natural prostacyclin has an enol ether bond, which is apt to be appreciably hydrolyzed, in its molecule, its action is readily paralysed under the neutral or acidic conditions, and accordingly it can not be a desirable compound to be used as medicine because of its chemical unstableness. To resolve such defect of natural prostacyclin, efforts have been directed to the development of chemically stable synthetic prostacyclin derivatives which have the same physiological activities as natural prostacyclin in Japan and abroad.
Especially, 6,9-(0)-methanoprostacyclin (Carbacyclin), which is a compound obtained by substituting the oxygen atom at the 6,9-positions or prostacyclin with a methylene group, is known as one of prostacyclins which are able to satisfy the chemical stableness to perfection (J. R. Vane et al., "Prostacyclin", pp. 31-41, Raven Press, N.Y., 1979) and is expected to work effectively as medicines. However, this 6,9(0)-methanoprostacyclin can not necessarily be regarded as a desirable compound in that it is inferior to natural prostacyclin in biological activity and it is not enough specific in action-selectivity. Beside this, nitroprostacyclin, a derivative obtained by substituting the oxygen atoms at the 6,9-positions with --N.dbd. group, is known as another type of stable prostacyclin and it is said that its biological activity is equal to that of natural prostacyclin (G. L. Bundy et al., Tetrahedron Letter, 1371 (1978) and W. Bartman et al., Tetrahedron Letter, 23, 3647 (1982)).